In the OVA, Masato loses his memories due to circumstances... and promptly falls in love with his mother. While this resulted in something of a Broken Base (consisting of those who preferred the show's Incest Subtext remain relegated to jokes and those who bought into the OVA's plot due to Rule of Funny), this is actually surprisingly plausible — Genetic Sexual Attraction posits that individuals who are related, yet never get close during their formative years, may actually develop sexual feelings despite (or perhaps because of) being genetically related. Masato had already said in the series that his mother was an objectively beautiful woman (while seeing her solely as his mother), so the amnesia simply erased the years he would have spent associating her as his Mom while getting desensitized to her.
